Positioning entities is done by specifying the offset on the x, y and z axis.
<a-box
position="0 2 -2"
></a-box>
If an entity is inside another entity, the positioning is relative to the parent entity.
<a-entity position="-2 2 -2">
<a-box position="-2 2 -2">
<!-- This box will actually be at -4 4 -4 relative to the scene-->
</a-box>
</a-entity>